The invention relates to method of operating a fuel cell and/or systems containing one or more fuel cells.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the gases (the anode gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas (the cathode gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode gas flows through the channels of the anode flow field plate, the anode gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode gas flows through the channels of the cathode flow field plate, the cathode gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode gas to catalyze the conversion of the anode gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode gas and the reaction intermediates to catalyze the conversion of the cathode gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the anode gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the cathode gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3.
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in equation 1, the hydrogen forms protons (H+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water or other low conductivity fluids) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
The invention relates to methods of operating a fuel cell and/or systems containing one or more fuel cells.
The portion of the membrane in a PEM fuel cell where a reactant gas is introduced into the membrane (i.e., the leading edge of the membrane) can begin to dry out if the reactant gas has a dew point below the temperature of the leading edge of the membrane. This can decrease the useful life of the membrane. It can therefore be advantageous for the reactant gas to have a dew point that is about the same as the leading edge of the membrane. This can be accomplished, for example, by hydrating the reactant gas before it enters the fuel cell. But hydrating the gas can result in liquid water collecting in the channels of the flow field plate, a situation commonly referred to as xe2x80x9cflooding,xe2x80x9d which can reduce fuel cell voltage performance.
The invention relates in part to the realization that switching the flow direction of a reactant gas through a flow field plate during a time period when the reactant gas is not flowing through the flow field plate can prolong the useful life of a PEM by reducing drying out of the leading edge of the PEM while also avoiding other potential situations that can reduce fuel cell performance.
One potential advantage is that the methods of the invention can be performed without hydrating the reactant gas so that flooding can be avoided.
Another potential advantage is that the methods of the invention can be performed without increasing the ratio of oxidant gas to fuel gas flowing through the fuel cell. This can avoid operating the fuel cell in electrolysis mode, which can be advantageous because the useful life of a PEM can be reduced as a result of operating the fuel cell in electrolysis mode.
In general, one aspect of the invention relates to a method of operating a fuel cell that includes flowing a reactant gas (e.g., an oxidant gas or a fuel gas) through the fuel cell in one direction so that the fuel cell produces a power output greater than zero. The flow of reactant gas through the fuel cell is then stopped so that the power output of the fuel cell is about zero.
The reactant gas is then flowed through the fuel cell in the opposite direction so that the power output of the fuel cell is greater than zero.
In some embodiments, the reactant gas flows through the fuel cell for a time period of at least about one week (e.g., at least about one month, at least about three months, at least about six months) before the flow of the reactant gas is stopped.
The method can further include, between stopping the flow of the reactant gas and flowing the reactant gas in the opposite direction, performing at least one maintenance step on the fuel cell. After flowing the gas in the reactant gas in the opposite direction, the reactant gas flow can be stopped so that the power output of the fuel cell is zero. After stopping the flow of the reactant gas through the fuel cell in the opposite direction, the reactant gas can flow through the fuel cell in the original direction.
The method can further include: flowing a different reactant gas through the fuel cell in one direction so that the fuel cell produces a power output greater than zero; stopping the flow of the different reactant gas through the fuel cell so that the power output of the fuel cell is about zero; and flowing the different reactant gas through the fuel in the opposite the first direction so that the power output of the fuel cell is greater than zero.
The method can further include: flowing a coolant through the fuel cell in one direction; stopping the flow of the coolant through the fuel cell; and flowing the coolant through the fuel in the opposite direction. The flow of the coolant can be stopped when the flow of the reactant gas is stopped. The flow of the reactant gas in the first and second directions can be concurrent with the flow of the coolant in the first and second directions In general, another aspect of the invention relates to a method of operating a fuel cell stack having two or more fuel cells. The method includes: (a) flowing a reactant gas through the fuel cells in one direction so that the fuel cell stack produces a power output of greater than zero; (b) stopping the flow of a reactant gas through at least one of the fuel cells so that the fuel cell(s) produce a power output of about zero; and (c) flowing the reactant gas through the fuel cells in the opposite direction so that the power output of the fuel cell stack is greater than zero. For the fuel cell(s) in which the reactant gas flow was stopped in step (b), the reactant gas flows through the fuel cell(s) in opposite directions in steps (a) and (c).
At least one maintenance step can be performed on the fuel cell stack between steps (b) and (c). The flow of reactant gas can be stopped after step (c), and then switched to the opposite direction.
During step (b), the reactant gas can flow through one or more of the other fuel cells so that the power output of the fuel cell stack is greater than zero. Alternatively, the flow of reactant gas through all the fuel cells can be stopped during step (b).
The power output of the fuel cell stack during step (b) can be about zero, or it can be greater than zero.
The method can further include: flowing a coolant through the first and second fuel cells in one direction; stopping the flow of the coolant through the first fuel cell; and flowing the coolant through the first and second fuel cells in the opposite direction. The flow of the coolant can be stopped when the flow of the reactant gas is stopped. The flow of the reactant gas in the first and second directions can be concurrent with the flow of the coolant in the first and second directions.
Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.